This section is intended to provide a background to the various embodiments of the invention that are described in this disclosure. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
The 3rd Generation Partner Project (3GPP) has discussed Coordinated-Multiple-Points (CoMP) as a typical case among existing multi-point diversity schemes. Among different modes for Downlink (DL) CoMP implementation, joint transmission is a one feasible way to increase diversity, i.e., multiple access points transmit the same signal to the same user equipment (UE) simultaneously with the same time-frequency resources. The signals from different access points are encoded so that they coherently combine in the air when they reach the UE. It is transparent to the UE that it is served by multiple points if a UE specific demodulation reference signal (DM-RS) is utilized and coherently transmitted by all access points. The signals from the different points should meet strict timing and phasing requirements so that the signals from different points are constructively combined. This has the effect of improving the signal to noise and interference ratio (SINR) at the UEs, and thus improves the robustness of the link.
A 5G system (e.g., microwave network) may work at higher radio frequency bands than a Long Term Evolution (LTE) system. Due to a high-frequency radio up to microwave frequency, many factors such as terminal rotation, several-meters-sized obstacles and UE mobility would lead to link quality fluctuations on different time scales. At the same time, higher requirements for both high reliability and low delay may be put on the 5G system due to specific traffic types. To meet this requirement, the suitability of higher frequency bands should be enhanced by transmission and/or reception diversity, such as multi-point diversity. In other words, diversity are needed to provide significant gain for 5G Radio Access Technologies, especially when working at higher frequency bands, e.g., above 6 GHz.
In 3G (e.g., UMTS (Universal Mobile Telecommunications System) Terrestrial Radio Access Network (UTRAN)) and 4G (e.g., Evolved UMTS Terrestrial Radio Access Network (E-UTRAN)) systems, the implementation of the multiple point connectivity is supported by the flow control technique, by which the duplicated downlink data transmissions from multiple access points to UE can be managed at network side. In the existing flow control solutions, the network relies on the UE measurement feedback to a relevant network node (e.g., Radio Network Controller (RNC) for UTRAN, a master evolved NodeB (MeNB) for E-UTRAN) to make the flow control decision, and then the result of the flow control decision is informed via backhaul to the access points that serve the UE.
However, in the 5G systems, the network-based flow control solutions become limited at least because of much weaker or shaky radio link in high-frequency spectrum and backhaul latency.
With increasing carrier frequency, the radio propagation becomes more shaky compared to lower frequency, due to for example, UE rotation, obstacles and UE mobility. Consequently, for the network-based flow control solutions that rely on UE measurement feedback, the successful transmission of UE measurement report to a particular access point (e.g., the master eNB) would be a premise of the flow control to take effect. When the transmission link between the UE and the access point crashes, the flow control function would not run successfully.
On the other hand, backhaul latency may also cause performance degradation. With the improvement of air interface capability discussed in 5G scope, the collision between high capability air interface and limited bandwidth in backhaul network become more and more clear. The latency for the network side to act on the channel quality change may consist of the following factors:                Backhaul latency: Iub/X2 round-trip time (RTT) for RNC/MeNB to get information from NodeBs (i.e., master NodeB and secondary NodeB (s))/secondary eNB (s) (SeNBs), and send packets to NodeBs/SeNB(s) based on the flow control decision;        Radio link latency: Radio Link Control Layer (RLC) RTT for NodeBs/SeNB to get an acknowledgement message from UE.        
With the improvement of 5G air interface, the radio link latency can be significantly reduced, but not for the backhaul latency, which may form the bottleneck that limits the flow control performance.
Therefore, there is a need to provide a new flow control solution for downlink multi-point diversity in 5G mobile communication networks.